1. Field of the Invention
The present invention relates to a failure diagnosis device for an engine capable of determining an abnormality of the engine such as a failure in an ignition system, a failure in a fuel system and so on.
2. Discussion of Background
The conventional failure diagnosis device for an engine determines the abnormality of the engine from the result of a comparison between a signal corresponding with a catalyst temperature output from a temperature sensor of the catalyst which is installed in an exhaust passage of the engine and purifies the exhaust gas of the engine, and a set value corresponding with a predetermined temperature.
For instance, when the engine misfires, unburnt fuel flows to the catalyst which causes a chemical reaction such as oxidation or deoxidation. As the result, a reaction heat is generated at the catalyst and the catalyst temperature is increased compared with that in the normal run of the engine. The catalyst temperature on this occasion is detected by the above temperature sensor and a failure of the engine is determined by the method as mentioned above. When the abnormality of the engine is determined, the failure diagnosis device for the engine enlightens a display lamp and informs the abnormality to a driver.
Since the conventional failure diagnosis device for an engine is composed as above, for instance, in the high load region of the engine, the air-fuel ratio of the engine is enriched, and unburnt fuel is contained in the exhaust gas as discharged to the air. The unburnt fuel touches the catalyst which causes a chemical reaction and elevates catalyst temperature. As the result, this type of failure is difficult to be differentiated from a failure in an ignition system. Therefore the accuracy of the failure diagnosis is deteriorated.
When the reference temperature is set to a high value to enhance the accuracy of the failure diagnosis, and the above temperature sensor detects a catalyst temperature which is higher than the predetermined reference temperature, the catalyst has already reacted with the unburnt fuel enough to be deteriorated by the heat, and cannot be used.
Furthermore in the conventional method, the detection of the catalyst temperature is not specific to each cylinder or to each group of cylinders. Therefore it is impossible to judge the abnormality of the engine with respect to each cylinder or each group of cylinders.